The invention relates to dynamic focus adjustment circuits for cathode ray tubes, particularly using an autotransformer for boosting the output voltage of a gated high voltage amplifier.
In cathode ray tubes for display or projection, one or more electron guns emit electrons that are electrostatically attracted to the phosphors on the faceplate of the cathode ray tube. The amplitude of the electron beam current is varied to vary phosphor luminance as the beam is scanned in a raster pattern by time varying magnetic fields produced by deflection coils. A beam of electrons tends to diverge, due to electrostatic repulsion. As the electrons in a CRT beam move from the cathode to the screen (anode), focusing devices associated with the electron gun cause the diverging electrons in the scanning beam to converge again at a focused point. The precise point of minimum beam diameter depends on the distance over which the converging or focusing forces operate. If the beam converges at a distance that is greater or less than the distance between the cathode and the instantaneous scanning point of the beam on the screen, the beam will be unfocused. More particularly, the beam will be wide, diffuse and incident on a larger area of the screen than it might be. The best possible resolution of the picture, which might otherwise have a pixel size as small as one phosphor dot, cannot be achieved unless the beam is focused so as to be incident on just one pixel.
The distance between the cathode and the point of minimum diameter can be considered the focal length of the focusing device. The focusing device operates in a manner that is similar to the operation of an optical lens to converge rays of light by diffraction. However, the focal length needed from the focusing device of a CRT varies during the scanning of the raster because the distance between the cathode and the screen is not constant over the area of the typical screen.
If all of the points on the screen were at equal distance from the electron gun, the screen would define a section of a sphere having an origin at the electron gun. Modern picture tube designs seek to limit the cabinet depth of a television receiver while having a relatively large display area, and the faceplates are generally very much flatter than the surface of such a sphere. The typical screen has a mild radius of curvature rather than being planar, but the radius of curvature is different from the distance between the electron gun and the screen. The typical radius of curvature is longer than the distance between the beam origin and the phosphors and not constant over the whole faceplate.
The CRT is provided with a variable focus circuit, that accommodates the difference in distance between the cathode and the screen at different points on the screen. The shortest radial distance is generally at the center of the screen. The radius is longer progressing laterally side to side, or vertically up and down, away from the center. The maximum distance is at the four corners of the display. Other layouts are possible, wherein one or more electron guns are off center.
A focus circuit applies an electrostatic voltage to converge or focus the beam at a point on the screen. The focusing forces include DC and AC components, for example using separate focus electrodes. To accommodate the difference in distance for different points on the screen, a dynamic focus voltage is also applied to vary the focal distance of the focus circuit during scanning of the raster. The required dynamic voltage is generally parabolic shaped, i.e., having a greater rate of change near the edges of the screen than in the center. In the case of an extremely wide and shallow picture tube, the difference in slope may be extreme, in which case the dynamic component may be called xe2x80x9cbathtub shaped.xe2x80x9d
It is known to derive a parabolic voltage component for focus modulation at the horizontal scanning rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage. It is also known how to provide a vertical rate parabola by integrating a sawtooth signal used in controlling vertical deflection. Thus, in order to drive the focus electrode(s) of the CRT, for example, a rectified DC level is obtained from the B+ voltage of the deflection power supply or from a winding of the flyback transformer, plus a horizontal rate parabola-shaped AC signal, plus a similarly shaped vertical rate parabola.
As another solution, it is known to use deflection yoke current coupled to a current transformer that is terminated by a capacitor to integrate the ramp of current representing horizontal deflection current. This provides a high voltage parabola. Such solutions, based on generating signals more or less directly from the deflection circuits, work well in some situations. However, they suppose substantially continuous signals, and do not work well, for example, in automatic kine bias (AKB) situations in which it is desirable to gate the focus signal on and off, or where the horizontal signal is otherwise something other than a continuous parabola.
U.S. Pat. No. 6,278,246xe2x80x94George and U.S. Pat. No. 6,118,233xe2x80x94Craig teach electrostatic focus circuits comprising high voltage amplifiers powered from deflection windings and operable to produce focus drive signals wherein the signals are gated on and off for AKB purposes. Gating off the dynamic focus signal at the low level during the AKB sampling interval ensures that current in the CRT is accurately sampled without contamination by internal coupling of the high voltage horizontal rate waveform. An amplifier has the advantage of facilitating such on/off gating of the dynamic focus waveform, and also provides some flexibility for optimizing the shape of the dynamic focus waveform, for example providing opportunities to shape the focus signal into a xe2x80x9cbathtubxe2x80x9d shape.
However, such an amplifier operates at a rather substantial voltage. These voltage levels are at the upper extremes of the range for which certain amplifier components, such as transistors, are available or will operate over a long useful life. It would be advantageous to provide a circuit having the benefits of a high voltage amplifier, but wherein there is less operational stress on the components, and less expense associated with the circuit.
For example, a design criteria for a television or other image processing device with a CRT may entail use of components that are rated for at least a given voltage or power level that is higher than the level at which such components are to be used, e.g., by some amount or proportion. The object of such design criteria might be, for example, to de-rate component parameters in order to extend the useful life of the apparatus. It is difficult or impossible, however, to employ such design criteria if the circuit design necessitates operations at the extreme of the range for which components are available.
According to an inventive aspect, the present invention reduces the need for components such as transistors, to operate at the upper extremes of available or rated voltage levels. That is, even if a component is available to operate at a given voltage level, according to the invention a lower voltage component can be used. This is accomplished by providing a step-up transformer coupled to the output of a dynamic focus amplifier, so as to increase the output voltage level while maintaining many of the other benefits associated with a high voltage amplifier type of focus drive circuit.